1. Field of the Invention
The present invention relates in general to fuels consumed by a gas-to-liquids (GTL) utilities unit. More specifically, the present invention is directed toward methods of adjusting the Wobble Indices of the fuels that provide the energy needs of a gas-to-liquids facility.
2. State of the Art
A gas-to-liquids (GTL) facility converts gaseous hydrocarbons into a wide variety of liquid hydrocarbon products ranging from naphtha to kerosene, diesel, and fuel oils. The starting material for these facilities can be natural gas, a fuel source that comprises predominantly methane, but which may also contain small amounts of higher analogs such as ethane and propane. One method of converting gaseous fuels such as natural gas into liquid fuels is known as the Fischer-Tropsch process. This process utilizes a reaction scheme that was developed in the early 1920s.
In the Fischer-Tropsch process, methane is first converted to a product called syngas, which is a mixture of carbon monoxide and hydrogen. Syngas may also contain components such as water, carbon dioxide, methane, higher hydrocarbons, nitrogen, and argon. The syngas is subsequently converted to the longer chain liquid hydrocarbons mentioned above. In practice, though, the syngas produced at a GTL facility is only partially converted into liquid hydrocarbons; the unconverted portion is commonly referred to as “tail gas.” Conventionally, the tail gas is frequently routed to a tubular steam reformer. The tail gas may be used as an energy source for a variety of the utilities needed to operate the GTL facility. These utilities include steam boilers, steam superheaters, electrical power generators, process steam heaters, and the like. Gas powered turbines used for electrical power generation are exemplary of the GTL utilities unit equipment that is very sensitive to changes in the Wobble Indices of its sustaining fuels.
In general, the two most common sources of fuel available to a GTL facility may be the natural gas asset itself, from which the feedstock syngas is produced for Fischer-Tropsch operations, and the tail gas that is a byproduct of those operations. Since natural gas comprises predominantly methane, and since the tail gas includes carbon oxide products that have a low (or zero) heating value, the heating value of the natural gas (and other burning properties such as Wobble Index) is higher than that of the tail gas.
It is advantageous to use tail gas as a source of energy for the GTL facility because to do so allows for a more efficient use of the natural gas resource. In some instances the natural gas asset itself is used for flaring, or otherwise disposing of combustible components, but this is an inefficient use of the resource. For these reasons, tail gas is an excellent choice of a fuel source for sustaining a GTL facility.
However, tail gas is not necessarily available to fuel the facility during certain times of its operation, such as startup, shutdown, and emergencies. During these periods, materials to fuel the facility must be obtained from alternative sources, and frequently the natural gas asset itself is used. Additionally, severe problems can arise if the burners and control systems that are designed to use fuel gas in normal situations are abruptly shifted to a fuel having a much different Wobble Index.
The problems associated with different Wobble Indices may be experienced no matter which direction the change is made; in other words, an increase in the Wobbe Index can be just as disastrous as a decrease in Wobbe Index. For example, if the Wobbe Index of a subsequent fuel is higher than the previous fuel the air supply to the burner may become the limiting factor to combustion, causing the flame temperature to drop and emissions to increase. If the controls are not designed properly, and if the furnace is not being monitored during these events, the rate of consumption of the fuel may actually increase even though a fuel with a higher Wobbe Index is being fed to the burners. The risks inherent with increased fuel consumption include fire and explosion.
On the other hand, if the Wobbe Index of the second fuel is significantly lower than that of the first fuel being consumed, which could happen if the facility switches to tail gas, the air supply to the furnace can exceed that which is required, causing a drop in furnace temperature. In this case the tail gas may then be only partially combusted, resulting in a release of carbon monoxide, and this can pose a serious threat to operators of the facility as well as members of the surrounding community.
One solution to the problem of widely variable Wobbe Indices of different fuels is to provide separate burners and separate fuel distribution lines for each of the types of fuels used by the facility. Alternatively, a burner with multiple burner tips may be employed to facilitate burning multiple fuels with varying Wobbe indices. It will be recognized by those skilled at the art, however, that this would be an expensive solution. It would be much more cost effective to devise methods of controlling or adjusting the Wobbe Index of each of these fuels, including tail gas, natural gas, and syngas, so that only one set of burners, furnaces, control systems, and fuel distribution lines are needed.
What is needed is a method of operating the utilities of a GTL facility such that more than one type of fuel may be used by the same burners and furnaces in the utilities unit. Also needed are methods of treating the fuels that sustain the facility, which may include methods of adjusting the Wobbe Index of the fuels, such that the GTL utilities may operate in a more safe and efficient manner.